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FIGURE 15.15 Although there may be as many as 1 trillion
comets in the Oort cloud, the volume of space that the Oort cloud
occupies is so immense that the comets are separated from one
another by distances that are typically about 10 AU.

a large cloud of invisible hydrogen gas surrounds the head, and
this hydrogen halo may be hundreds of thousands of km across.
As the comet nears the Sun, the solar wind and solar ra-
FIGURE 15.16 As a comet nears the Sun, it grows brighter,
diation ionize gases and push particles from the coma, pushing with the tail always pointing away from the Sun.
both into the familiar visible tail of the comet. Comets may have
two types of tails: (1) ionized gases and (2) dust. The dust is
pushed from the coma by the pressure from sunlight. It is visible have long elliptical orbits that return them at intervals exceed-
because of reflected sunlight. The ionized gases are pushed into ing 200 years. The famous Halley’s comet has a smaller ellipti-
the tail by magnetic fields carried by the solar wind. The ionized cal orbit and returns about every 76 years. Halley’s comet, like
gases of the tail are fluorescent, emitting visible light because all other comets, may eventually break up into a trail of gas
they are excited by ultraviolet radiation from the Sun. The tail and dust particles that orbit the Sun.
generally points away from the Sun, so it follows the comet as it The Stardust spacecraft had a mission to fly to a comet named
approaches the Sun but leads the comet as it moves away from P/Wild 2 and collect samples of dust and volatiles from the coma
the Sun (Figure 15.16). of the comet. It returned these samples to Earth, along with in-
Comets are not very massive or solid, and the porous, terstellar dust samples collected on the way to the comet. These
snowlike mass has a composition more similar to that of the samples represent primitive substances from the early formation
giant planets than to the terrestrial planets in comparison. of the solar system and continue undergoing detailed analysis.
Each time a comet passes near the Sun, it loses some of its
mass through evaporation of gases and loss of dust to the solar
wind. After passing the Sun, the surface forms a thin, fragile ASTEROIDS
crust covered with carbon and other dust particles. Each pass Between the orbits of Mars and Jupiter is a belt, or circular re-
by the Sun means a loss of matter, and the coma and tail are gion, of thousands of small, rocky bodies called asteroids (Fig-
dimmer with each succeeding pass. About 20 percent of the ure 15.17). This belt contains thousands of asteroids that range
approximately 600 comets that are known have orbits that re- in size from 1 km or less up to the largest asteroid, named Ceres,
turn them to the Sun within a 200 year period, some of which which has a diameter of about 1,000 km (over 600 mi). The as-
return as often as every 5 or 10 years. The other 80 percent teroids are thinly distributed in the belt, 1 million km (about

15-15 CHAPTER 15 The Solar System 391